The human heart may surlier from two classes of rhythmic disorders or arrhythmias: Bradycardia and tachyarrhythmia. Bradycardia occurs when the heart beats too slowly, and may be treated by a common pacemaker delivering low voltage (about 1 V) pacing pulses. Of concern here is tachyarrhythmia, which involves an abnormally high heart rate between about 100 to 200 beats per minute, but without hemodynamic or blood flow efficiency. Of particular concern is a ventricular tachycardia, in which the ventricles have not completely filled before they contract, thus diminishing the voltime of blood pumped. The pumping inefficiency is generally proportional to the heart rate. A severe form of tachyarrhythmia is fibrillation, which occurs at heart rates of 180 to 300 beats per minute, and involves erratic, disorganized beating that pumps virtually no blood.
Implantable cardioverters/defibrillators (ICD) or pulse generators are used for antitachycardia pacing to correct rapid heart rates by delivering a rapid sequence of pacing pulses of 1 to 10 volts to break the arrhythmia. ICD devices treat severe tachycardia with cardioversion, by delivering a shock of 100 to 750 volts synchronously with the peak of the heart's R-wave signal as detected by an electrocardiogram (ECG). Heart fibrillation receives similar therapy, but the erratic ECG signal may not provide a clear R-wave peak for synchronization.
Normally, the spacing between successive R-wave peaks is used to determine the heart rate. Extremely high or irregular heart rates clearly require therapy. Moderately elevated heart rates may be of ambiguous origin, either from healthy exercise, or from the disorders discussed above. To distinguish between these causes, treatment techniques have included measurement of blood pressure, oxygen saturation, Doppler ultrasound parameters, and ECG morphology. These techniques have limited accuracy and practicality, particularly outside of a clinical setting.
The present invention avoids the limitations of existing techniques and devices by providing a cardiac blood flow sensor that measures blood flow within the heart. The apparatus includes a light source and a photodetector within an implanted housing. The light source projects a beam through a flexible elongated light conduit having a first end optically connected to the housing and having a distal tip positioned within the patient's heart. The distal tip includes a ruby having fluorescent characteristics that vary with temperature. The temperature of the ruby is determined initially from the ruby fluorescence decay time. Then the beam is activated for a period of time to elevate the temperature of the ruby well above the ambient blood temperature. The temperature of the ruby is again determined from its fluorescence decay time. The elevated temperature difference from the initial temperature depends on the cooling effect of the blood flow. The cooling effect is greater during healthy blood flow than during tachyarrhythmia.
The fluorescent light is transmitted back through the conduit to the photodetector, which generates a signal that may be analyzed by a controller to determine whether a tachycardia is physiologic or pathologic in origin.
The flow sensor may be contained in a common housing with a defibrillator that is implanted in a patient. The sensor may remain inactive until a tachycardia is detected, upon which the light source is activated. The defibrillator may be activated only if the flow sensor has detected a blood flow rate below a predetermined level.